1. Field
Apparatuses and methods consistent with exemplary embodiments relate to a synchronization system that provides synchronization among a plurality of terminals connected to a network.
2. Description of the Related Art
A content sharing service has been proposed in which communication such as a telephone call is performed while content, such as a moving image, is simultaneously viewed among a plurality of terminals connected to a network. In such a content sharing service, if a moving image is simultaneously viewed among the plurality of terminals, reactions (laughs or surprises) to the viewed moving image, and a conversation in the telephone call become out of synchronization unless the same scene (in more detail, the same frame) of the moving image is displayed at the same time. In other words, a reproduction position, a stop position or the like of the simultaneously viewed moving image needs to be synchronized among the plurality of terminals. Here, a phenomenon in which the reproduction position or the stop position shifts among the plurality of terminals is hereinafter referred to as “error of synchronization”.
A cause for occurrence of a problem of “error of synchronization” as described above exists in a network delay in transmission of a control signal for starting reproduction, stopping the reproduction or the like, to the plurality of terminals connected to the network. Specifically, an amount of the network delay (One-Way Delay Time) from a control signal transmitter (for example, a control server) to each terminal is greatly different depending on a network route thereof. Thus, even if the control signal has been transmitted to each terminal at the same time, a time when this control signal arrives at each terminal becomes different. As a result thereof, even if each terminal performs reproduction control according to the control signal, the error of synchronization occurs.
On the other hand, for the problem of the error of synchronization as described above, in a synchronization method described in Patent Literature 1, there is proposed a method of achieving synchronization by referring to a highly-accurate global clock (such as an NTP) from the plurality of terminals. Moreover, in a synchronization method described in Patent Literature 2, there is proposed a method of achieving synchronization by repeating a method of estimating the One-Way Delay Time from the control server to each terminal, based on ½ of an RTT (Round Trip Time=a sum of the One-Way Delay Time from the control server to the terminal, and the One-Way Delay Time from the terminal to the control server) (Technical Specification RFC (Request for Comments) 2030).
However, in the above described synchronization method described in Patent Literature 1, there is a problem that a global clock that can be referred to from all the terminals synchronously viewing the content and is highly accurate needs to exist, and if such a global clock cannot be prepared, this method is not applicable.
Moreover, in the above described synchronization method described in Patent Literature 2, a problem occurs in which if the One-Way Delay Time from the control server to each terminal greatly deviates from ½ of the RTT (if One-Way Delay Times of a going way and a returning way greatly deviate), synchronization accuracy decreases. Moreover, in this synchronization method, a method of increasing the synchronization accuracy by measuring the RTT more than once is employed. However, a problem also occurs in which a time for the measurement is required many times, and the measurement time is required for each reproduction control of the content. Furthermore, in a network in which there is a steady-state deviation in the One-Way Delay Time between the control server and a certain terminal (for example, the One-Way Delay Time from the control server to the terminal is always larger than the One-Way Delay Time from the terminal to the control server, or the like), a problem occurs in which the accuracy is not improved even by measuring the RTT more than once.
In addition, in both of the above described synchronization methods described in Patent Literature 1 and Patent Literature 2, a problem also occurs in which the more the number of the terminals synchronously viewing the content increases, the larger a maximum error of synchronization value (for example, a difference between a terminal that starts reproduction at the earliest time and a terminal that starts the reproduction at the latest time) stochastically becomes.
Consequently, an object is to provide a synchronization system in which highly-accurate synchronization is enabled in a short time even if the global clock, that can be referred to from all the terminals and is highly accurate, does not exist, and the decrease in the synchronization accuracy can be suppressed even if the number of terminals increases, which are the above described problems.